1. Field of the Invention
This invention relates to an apparatus for vapor phase epitaxial growth on the substrate surface by introducing a raw material gas or gas mixture for thin film formation onto the surface of a heated substrate, and in particular, to an apparatus for vapor phase epitaxial growth in which a grown layer uniform in film thickness can be obtained on the substrate, and to a method of substrate heating.
2. Description of the Background Art
As schematically illustrated in FIG. 6, a typical conventional apparatus for vapor phase epitaxial growth comprises a reactor tube 1 made of quartz, a holder 2 disposed within the reactor tube 1, a susceptor 3 supported by the holder 2 and a high-frequency heating coil 5. The susceptor 3 is made of, for example, carbon and is heated to a desired temperature by means of the high-frequency heating coil 5. A substrate 4 is placed on the susceptor 3 and is heated to a desired temperature by heat conduction from the susceptor 3.
For convenience of illustration, let it be supposed that a GaAs (gallium arsenide) substrate is used as the substrate 4, that a mixed gas composed of trimethylgallium (TMG) gas, trimethylaluminum (TMA) gas and arsine (AsH.sub.3) gas is used as the raw material gas mixture for epitaxial growth and that hydrogen (H.sub.2) gas is used as a carrier gas. It is also supposed that diethylzinc gas is used as a dopant. The GaAs substrate 4 is heated to a temperature of, for instance, about 800.degree. C. by heat conduction from the susceptor 3. The raw material gas mixture introduced into the reactor tube 1 is thermally decomposed on the heated GaAs substrate 4 and, as a result, epitaxial growth of an AlGaAs (aluminum gallium arsenide) layer takes place on the substrate 4. Then, the supply of TMA gas is discontinued, and a mixed gas composed of TMG gas and AsH.sub.3 gas is introduced into the reactor tube 1 as the raw material gas mixture, whereby epitaxial growth of a GaAs layer takes place on the AlGaAs layer on the substrate 4. Repetition such steps can result in epitaxial growth of a number of layers on the substrate 4.
Prior to epitaxial growth, the GaAs substrate 4 is, for example, mirror-finished on the face side and lapped on the reverse side. Thus, the face side and the reverse side of the substrate 4 differ in finished surface smoothness in many instances. In certain instances, the substrate 4 is subjected to patterning on the face or reverse side or already has thereon an epitaxial layer differing in thermal expansion coefficient from the substrate 4. In such instances, when heated to a temperature of about 800.degree. C., the substrate 4 curls in a concave or convex fashion as illustrated in FIG. 7, since the face and reverse sides differ in condition. As a result, a clearance 6 is formed between substrate 4 and susceptor 3.
As mentioned above, the substrate 4 is heated to a desired temperature by the conduction of heat from the susceptor 3. When the substrate curls to give rise to a clearance 6 between substrate 4 and susceptor 3 as shown in FIG. 7, a temperature gradient or non-uniformity in the planar direction is created within substrate 4. In other words, whereas that portion of substrate 4 which is in contact with susceptor has a relatively high temperature, the portion apart from the susceptor has a relatively low temperature. A non-uniform temperature distribution on the surface of substrate 4 leads to the formation on the substrate of an epitaxially grown layer which is locally varying in thickness.
Clearances 6 are also formed between the substrate 4 and susceptor 3 when the reverse side of substrate 4 and/or the face side of the susceptor 3 is uneven, as shown in FIG. 8. In this case, too, an uneven temperature pattern results within the substrate 4 in the planar direction and the epitaxially grown layer formed on the substrate 4 becomes uneven in film thickness.